Liquid crystal devices

ABSTRACT

A ferroelectric liquid crystal display (FLCD), which comprises a cell including a layer of chiral smetic ferroelectric liquid crystal material contained between two substrates and at least one alignment layer for determining the surface alignment of the molecules in the liquid crystal material, is manufactured as follows. After selection of a suitable material for the alignment layer, the cell is filled by introducing liquid crystal material between the substrates to which the alignment layer is applied. After filling of the cell, a heat treatment is applied to the cell by raising the cell to an elevated temperature and maintaining the cell at that temperature for a predetermined period of time. The temperature and the period of time of this heat treatment are selected to cause the surface alignment properties of the alignment layer to be changed by the heat treatment in such a manner as to promote adoption of the C2 state by the liquid crystal material on subsequent cooling to the device operating temperature.

This is a division of application Ser. No. 08/963,148, filed Nov. 3,1997 now U.S. Pat. No. 6,020,947.

BACKGROUND OF THE INVENTION

This invention relates to liquid crystal devices, such as ferroelectricliquid crystal devices, and methods of manufacturing such devices.

The surface stabilised ferroelectric liquid crystal device (FLCD)possesses the advantage over other liquid crystal devices, such as thetwisted nematic liquid crystal device, that it is a bistable devicewhich can be switched between two states by switching pulses ofalternate polarity and which will remain in one state in the absence ofa switching pulse until a switching pulse of appropriate polarity isapplied to switch it to the opposite state. By contrast, in a twistednematic liquid crystal device, a drive signal must be appliedcontinuously to maintain the device in one of its states.

A conventional FLCD cell comprises a layer of ferroelectric smecticliquid crystal material contained between two parallel glass substratesprovided on their inside surfaces with electrode structures in the formof row and column electrode tracks which cross one another to form anaddressable matrix array. Furthermore each of the inside surfaces of thesubstrates is provided with a suitable alignment layer which, prior toassembly of the substrates and filling of the cell with liquid crystalmaterial, is treated by rubbing to impart a preferred surface alignmentdirection, and preferably a surface pretilt, to the contacting moleculesof the liquid crystal material layer.

The switching behaviour of the liquid crystal molecules is dependent onthe arrangement of the molecules in microlayers which, in the case ofchiral smectic material, extend transversely of the substrates and adopta chevron geometry having two possible states, C1 and C2, as disclosedin J. Kanbe et al, Ferroelectrics (1991), vol. 114, pp. 3. Both C1 andC2 states can form as the material cools down from the isotropic phaseto the chiral smectic phase during device manufacture, and theboundaries between these two states may be seen as a zigzag defect. Whenused in a display device, material incorporating both the C1 and the C2states can appear patchy, and it is therefore preferred that thematerial should be in one state for a practical device. The C2 state ispreferred as it allows faster switching at lower voltages. Accordinglyit is important that both the alignment layers provided on thesubstrates have surface alignment properties which are such as topromote formation of the C2 state on cooling of the liquid crystalmaterial layer during manufacture of the device.

However little or no formation of the C2 state may occur with someliquid crystal materials when using a conventional device manufacturingmethod in which a bath of liquid crystal material is heated to atemperature at which the material is in the isotropic phase, the liquidcrystal material is drawn under vacuum between the substrates of thecell, and the cell is then cooled down slowly so that the materialpasses from the isotropic phase through the cholesteric and smectic Aphases to the chiral smectic phase. Furthermore the C2 state may beunstable with temperature so that the proportion of the material in theC2 state may vary with temperature.

In a colour FLCD, such as may be used in a colour display, one of thesubstrates of the cell may incorporate a colour filter layerincorporating red, green and blue areas for each pixel of the cell.During manufacture of such a device the colour filter layer is appliedprior to the application of the alignment layer to the substrate, andthis imposes a limit to the temperature of the subsequent heat curingtreatment which may be applied to polymerise and harden the alignmentlayer after spinning down of a liquid monomer on the substrate surfaceto form the alignment layer in known manner. Whereas the heat curingtreatment may take place at a temperature of up to about 300° C. in acell in which a colour filter layer is not provided, the heat curingtreatment must generally take place at a temperature less than 180° C.in a cell in which such a colour filter layer is provided, in order notto adversely affect the colour filter layer. However such a lowertemperature heat curing treatment may be insufficient to prevent thesurface alignment properties of the alignment layer being significantlychanged by heat treatments applied during further processing.

Furthermore spacer walls may be formed on at least one of the substratesfor spacing the substrates apart when the substrates are connectedtogether and for securing the substrates together over the entiresurface area of the cell. Such spacer walls may be formed by anadditional manufacturing step carried out prior to application of thealignment layer to the substrate, the additional manufacturing steptypically comprising spinning down of a polyimide layer on the substrateand selective etching of the layer to form the spacer walls at therequired locations. Subsequent to the formation of the spacer walls, thealignment layer is applied and rubbed to impart a preferred alignmentdirection, although the existence of the spacer walls can mean that itis difficult to properly rub all parts of the alignment layer.Furthermore the substrates are connected together by a heat bondingprocess, at a temperature of 150-180° C. for example, in order to bondthe spacer walls on one of the substrates to the surface of the othersubstrate, and such heat bonding can significantly change the surfacealignment properties of the alignment layer on the two substrates. If alower temperature heat curing treatment as described above haspreviously been applied to one of the substrates, for example becausethe substrate incorporates a colour filter layer, such heat bonding canaffect the surface alignment properties of the two alignment layers todifferent extents, thus producing the undesirable result that the twoalignment layers have significantly different surface alignmentproperties in the manufactured device.

It is an object of the invention to provide an improved method ofmanufacturing a liquid crystal device, for example by promoting the C2state in a ferroelectric liquid crystal device during manufacture.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method ofmanufacturing a ferroelectric liquid crystal device which comprises acell including a layer of chiral smectic ferroelectric liquid crystalmaterial contained between two substrates and at least one alignmentlayer for determining the surface alignment of the molecules in theliquid crystal material, the method including the steps of:

(a) selecting a suitable material for said alignment layer of the cell;

(b) filling the cell by introducing liquid crystal material between thesubstrates of the cell to which said alignment layer is applied; and

(c) after filling of the cell, applying a heat treatment to the cell byraising the cell to an elevated temperature and maintaining the cell atsaid temperature for a predetermined period of time, said temperatureand said period of time being selected to cause the surface alignmentproperties of said alignment layer to be changed by the heat treatmentin such a manner as to promote adoption of the C2 state by the liquidcrystal material on subsequent cooling to the device operatingtemperature.

The application of such a heat treatment after filling of the cell withthe liquid crystal material constitutes a high temperature annealingstep which serves to increase or decrease the surface alignmentproperties, such as the pretilt, imparted by the alignment layer so thatthe C2 state becomes more stable below the chiral smectic transitiontemperature. This not only promotes the formation of the C2 state, butalso ensures that the C2 state remains substantially unaffected bysubsequent variations in temperature. Furthermore certain liquid crystalmaterials may be caused to adopt the C2 state which would not otherwiseform the C2 state in use of a conventional device manufacturing method.

The invention further provides a method of manufacturing a liquidcrystal device which comprises a cell including a layer of liquidcrystal material contained between two substrates each of which isprovided with an alignment layer for determining the surface alignmentof the molecules in the liquid crystal material, the method includingthe steps of:

(a) applying a first alignment layer to a first substrate;

(b) applying a second alignment layer to a second substrate;

(c) heat treating the first and second substrates to cure the first andsecond alignment layers;

(d) rubbing the first and second alignment layers to provide therequired surface alignment;

(e) connecting the first and second substrates together so that thefirst and second alignment layers face one another with a gabtherebetween; and

(f) filling the cell by introducing liquid crystal material between thefirst and second substrates;

wherein, in step (c), different heat treatments having different effectson the surface alignment properties of the first and second alignmentlayers are applied to the first and second substrates, and the first andsecond alignment layers are made of different materials and/or aredifferently processed in order to compensate for the effects of thedifferent heat treatments so that the first and second alignment layershave similar surface alignment properties in the manufactured device.

By making the first and second alignment layers of different materialsand/or by processing the alignment layers differently, similar surfacealignment properties may be imparted to the first and second alignmentlayers even though different heat treatments are applied to thealignment layers. Thus, for example, if the first substrate incorporatesa colour filter layer, a relatively low temperature heat treatment ofless than 180° C. can be applied to the substrate to cure the firstalignment layer whilst not adversely affecting the colour filter layer,whereas, if spacer walls are to be applied to the second substrate, arelatively high temperature heat treatment at a temperaturesubstantially greater than 180° C., for example at a temperature ofabout 300° C., can be applied to the second substrate to fully hardenthe second alignment layer. Such high temperature curing of the secondalignment layer enables the application of spacer walls on top of thesecond alignment layer by a process in which a layer of material isapplied to the substrate and is subsequently etched, without the secondalignment layer being removed by the etching treatment. This allowsrubbing of the second alignment layer to impart a preferred alignmentdirection prior to application of the spacer walls, thus enabling thesecond alignment layer to be rubbed more uniformly than would bepossible if the second alignment layer was applied on top of the spacerwalls.

The invention also provides a liquid crystal device including a cellcomprising a layer of liquid crystal material contained first and secondsubstrates provided with first and second alignment layers fordetermining the surface alignment of the molecules in the liquid crystalmaterial, wherein the first and second alignment layers are made ofdifferent materials have inherently different surface alignmentproperties prior to heat treatment, different heat treatments havingbeen applied to the first and second substrates during manufacture tocause the first and second alignment layers to have substantiallysimilar surface alignment properties in the manufactured device.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more fully understood, preferredmethods in accordance with the invention will now be described, by wayof example, with reference to the accompanying drawings, in which:

FIG. 1 diagrammatically shows a section through a FLCD cell;

FIG. 2 is an explanatory diagram illustrating the two chevron states ofthe liquid crystal material in the FLCD cell;

FIG. 3 is a graph of the surface alignment properties of the alignmentlayer against the product of time and temperature of an annealingtreatment in such a cell; and

FIG. 4 is a diagrammatic section through part of a colour FLCD cell.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical structure of a FLCD cell 1 in which aferroelectric liquid crystal material 2 in the chiral smectic phase iscontained between two glass substrates 3 and 4 arranged parallel to oneanother and sealed at their edges. Transparent ITO (indium tin oxide)electrode structures 5 and 6 are applied to the inwardly directed facesof the substrates 3 and 4. Each of the electrode structures 5 and 6 isin the form of electrode tracks arranged parallel to one another, thetracks of the structure 5 being arranged in rows and the tracks of thestructure 6 being arranged in columns extending perpendicularly to therows so as to enable pixels at the intersections of the rows and columnsto be addressed by the application of suitable strobe and data pulses tothe intersecting tracks of the two electrode structures 5, 6.

A thin polymer alignment layer 7 or 8, for example a polyamide orpolyimide alignment layer, is applied to the inwardly directed face ofeach electrode structure 5 or 6, each alignment layer being treated toprovide a pretilt angle ξ of about 2° to 10° to the surface, forexample, and being rubbed in a required rubbing direction by buffingwith a soft cloth made of rayon, for example, in order to impart apreferred alignment to the molecules of the liquid crystal material 2 inthe vicinity of the alignment layers 7, 8. The rubbing directions of thetwo layers 7, 8 are typically parallel and in the same direction. As iswell known the liquid crystal material is aligned during manufacture bycooling through the higher temperature phases to the required chiralsmectic phase. When in the chiral smectic phase, the molecules areuniformly aligned in microlayers extending perpendicularly to the glasssubstrates 3, 4, the molecules in each microlayer adopting a chevrongeometry due to the alignment of the molecules at the surfaces of thesubstrates 3, 4 on the two sides of the liquid crystal layer, andpreferentially being aligned in the C2 state as referred to above,rather than in the C1 state.

The C1 and C2 states are shown diagrammatically in FIG. 2, the molecules20 being shown in each case aligned in a microlayer 21 between the innersurfaces 22 and 23 of the cell substrates in the appropriate one of thetwo chevron states, which differ in the angles made at the chevroninterface 24 midway between the surfaces 22 and 23. In this diagram themolecules 20 in each microlayer 21 are shown as if each molecular axisis on the surface of a cone 25 with the director 26 of each moleculebeing orientated at an appropriate angle in the plane of the base 27 ofthe cone 25. Strong aligning forces anchor the molecules 20 in a tiltedand aligned direction adjacent to each of the substrate surfaces 22 and23, the direction of tilt and the alignment direction being determinedby the surface properties of the treated alignment layer, whereas themolecules 20 away from the substrate surfaces tend to arrange themselvesin one of two stable positions on the surface of the cone 25. When asmall d.c. electric field of appropriate polarity, amplitude and time isapplied across the cell during switching by the data and strobe pulses,the molecules 20 rotate from one stable position on the surface of thecone 25 to the other stable position. The angle of the cone 25 aroundwhich each molecule 20 rotates is the cone angle θ, the angle betweenthe surface 22 or 23 and the microlayer 21 is δ, and the surface tilt orpretilt angle of the molecules at the surface 22 or 23 is ξ. In the C2Ustate, which is the preferred form of the C2 state, the director profileof the molecules has mirror symmetry about the central plane of thecell. In other words, the chevrons are symmetric and both surfaces areof the same orientation.

FIG. 3 is a graph illustrating the effect of annealing of the cell onthe surface alignment properties, that is the surface pretilt angle ξand the anchoring coefficients, of the alignment layer of the cell. Themanner in which the surface alignment properties vary with the degree ofannealing, that is the product of the temperature applied and theduration for which the temperature is applied, depends on the particularpolymer chosen for the alignment layer and the treatment of thealignment layer. For example, it has been shown that the surface pretiltangle of five different polymers, that is the polymers SE130, SE7311, SE4110 and SE610 supplied by Nissan and the polymer Probimide 32 suppliedby Ciba-Geigy, vary in different ways with variation in the annealingtime/temperature. In each case the pretilt angle was measured with thecell filled with liquid crystal material and after annealing and coolingto room temperature. Generally speaking, different polymers exhibitthree different types of behaviour, and these are shown in FIG. 3 by thethree curves a), b) and c). In the case of a) type polymers, the surfacealignment properties, such as the pretilt angle, remain substantiallyconstant with variations in the annealing time/temperature, whereas, inthe case of b) type polymers, the pretilt angle increases withincreasing annealing time/temperature and, in the case of c) typepolymers, the pretilt angle decreases with increasing annealingtime/temperature.

In a conventional FLCD manufacturing process the cell is filled byplacing it above a bath of liquid crystal material heated to atemperature at which the material is in the isotropic phase, so that thebottom edges of the substrates are in contact with the material withinthe bath, and by then applying a vacuum so as to slowly draw the liquidcrystal material upwardly between the substrates by capillary action,with the cell being maintained at a raised temperature during suchfilling. After filling of the cell, which may take a number of hours,the heat is removed and the cell is cooled down very slowly so that theliquid crystal material passes from the isotropic phase through thecholesteric and smectic A phases to the chiral smectic phase (usuallythe chiral smectic C phase) as the material cools. However one or moreof these phases may be omitted in certain liquid crystal materials.Furthermore the polymer selected for the alignment layer and the surfacetreatment imparted prior to assembly and filling of the cell areselected to favour the C2 state on cooling of the liquid crystalmaterial. Low or medium values of the pretilt angle tend to favour theC2 state, whereas high values of the pretilt angle tend to favour the C1state. The relevant criteria are described in more detail in J. C.Jones, M. J. Towler, J. R. Hughes, "Fast, high contrast ferroelectricliquid crystal displays and the role of dielectric biaxiality", Displays(1993), vol. 14, no. 2, pp.86.

However the application of heat to the cell in such a process willitself change the surface alignment properties of the alignment layer,particularly where the alignment layer is formed from a polymerexhibiting b) or c) type behaviour as described above. Furthermore, thefact that the cell is filled progressively from bottom to top when thefilling process referred to above is used means that the alignment layerwill be annealed to a greater extent in the lower part of the cell thanin the upper part of the cell, with the result that the surfacealignment properties of the alignment layer will differ from the bottomto the top of the cell. This means that the surface alignment propertiesof the alignment layer may vary unpredictably after the manufacturingprocess has been completed with the result that the proportion of theliquid crystal material in the C2 state will also vary. This can meanthat certain polymers, which otherwise exhibit desirable properties forpromoting the formation of the C2 state in a FLCD, are ruled out ofconsideration for the alignment layer because they exhibit type b) ortype c) behaviour as described above.

The invention proposes providing an annealing step in the manufacturingprocess so as to ensure that, whether the polymer used for the alignmentlayer is of type a), type b), or c), the surface alignment properties inthe manufactured cell are such that the surface tilt angle ξ≧θ-|δ| is ina preferred band B centred on ξ=2.5°, say extending between 2° and 3°,favouring formation of the C2 state. If, for example, a b) type polymeris selected for the alignment layer which is such as to provide apretilt angle of substantially less than 2.5° when the cell has beenfilled, the annealing step may be applied at a temperature and for atime sufficient to increase the pretilt angle in the manner indicated bythe curve b) in FIG. 3 until an optimum value is reached ensuring thatthe pretilt angle in the manufactured cell is as close as possible to2.5° at the operating temperature. Conversely, if a c) type polymer isselected for the alignment layer which is such as to provide a pretiltangle of substantially more than 2.5° after filling of the cell, theannealing step may be applied at a temperature and for a time sufficientto decrease the pretilt angle in the manner indicated by the curve c)until an optimum value is reached ensuring that the pretilt angle is asclose as possible to 2.5° in the manufactured cell at the operatingtemperature.

Thus, in a typical manufacturing process incorporating the annealingstep of the invention, the cell is assembled in conventional mannerafter the application of alignment layers to the inside surfaces of thetwo substrates and after treatment of the alignment layers as describedabove, the polymer for the alignment layers being selected to have theappropriate surface alignment properties in the manufactured cell. Afterfilling of the cell with liquid crystal material from a bath of liquidcrystal material at an elevated temperature in the manner alreadydescribed, the filled cell is subjected to the required annealing step,optionally after allowing the cell to cool to a temperature below thechiral smectic phase transition temperature, possibly even to roomtemperature. The annealing step may comprise heating the filled cell toa temperature at which the liquid crystal material is in the nematic orisotropic phase, for example a temperature of the order of 130° C., andthen maintaining the cell at this temperature for the required period oftime, for example one hour, to effect annealing of the cell. Thetemperature and duration of the annealing step will depend on theparticular polymer used for the alignment layer, as well as on thesurface alignment properties required in the manufactured cell, whichwill in turn depend on the particular liquid crystal material used inthe cell. Typically the temperature will be in the range of 100° C. to180° C., and the duration will be in the range of five minutes to threehours. When the annealing step has been completed, the heat is removedfrom the cell and the cell is slowly cooled, for example at a rate of 1°C. per minute, until the cell is at room temperature. At the completionof the cooling step the alignment layer has a surface pretilt angle, forexample of about 2.5°, which is matched as closely as possible to theideal angle for promoting formation of the C2V state in the liquidcrystal material, so that the molecules of the liquid crystal materialwill tend to align themselves in the C2V state during cooling.

A further manufacturing method in accordance with the invention will nowbe described with reference to the manufacture of a FLCD for use in acolour display which is shown diagrammatically in FIG. 4 and includes acell 1' comprising a layer of chiral smetic ferroelectric liquid crystalmaterial 2' contained between first and second substrates 3' and 4', thefirst substrate 3' incorporating a colour filter layer 15 and the secondsubstrate 4' having spacer walls 14 applied thereto. However it shouldbe understood that other types of liquid crystal device, which may ormay not incorporate a colour filter layer or spacer walls, may also beproduced by this further method. Furthermore the spacer walls 14 may bereplaced by spacer beads or active spacers, or the spacers may beproduced prior to over-coating with an alignment layer.

In order to manufacture the illustrated cell 1', the colour filter layer15, incorporating red R, green G and blue B areas corresponding to eachpixel, is applied to the first substrate 3' in known manner, andthereafter three further layers are applied to the first substrate 3',namely a planarisation layer 16, an electrode structure 17 in the formof column tracks of transparent indium tin oxide, and a barrier layer18, prior to the application of a first alignment layer 7'. No colourfilter layer or planarisation layer is applied to the second substrate4', although an electrode structure 19, in the form of row electrodetracks of transparent indium tin oxide, and a barrier layer 13 areapplied in similar manner to the first substrate, prior to theapplication of the second alignment layer 8'.

Each of the first and second alignment layers 7' and 8' is applied byspinning down a layer of liquid monomer is known manner and bysubsequently polymerising and hardening the layer by means of a heattreatment. However, instead of applying the same heat treatment to bothalignment layers 7', 8' as would normally be expected, the first andsecond substrates 3' and 4' are subjected to different heat treatmentsso that the alignment layers 7' and 8' are cured to different extents.In the case of the first substrate 3' incorporating the colour filterlayer 15, a relatively low temperature heat treatment at a temperatureof less than 180° C. is applied for a relatively short period of time,for example half an hour, in order not to adversely affect the colourfilter layer 15, whereas, in the case of the second substrate 4' towhich the spacer walls 14 are to be applied, a relatively hightemperature heat treatment at a temperature of about 300° C. is appliedfor a relatively long period of time, for example one to three hours.Each of the alignment layers 7', 8' is then rubbed, by buffing with asoft cloth made of rayon for example, to impart a preferred alignmentdirection to the layer in known manner.

The relatively high temperature heat treatment applied to the secondsubstrate 4' serves to harden the second alignment layer 8' to such anextent as to enable the spacer walls 14 to be applied on top of thealignment layer 8' by means of a process in which a layer of a liquidpolymer, such as polyimide, of a thickness corresponding to the requiredspacing apart of the substrates (typically 1500 nm) is spun down on thesubstrate, the layer being subsequently polymerised in known manner, forexample by exposure to ultraviolet radiation through a mask, prior tobeing etched to remove the material of the layer in those areas in whichspacer walls are not provided. This process leaves the material inposition in those areas in which the spacer walls 14 are provided whilstensuring that the hardened alignment layer 8' is not attacked by theetching treatment.

After processing of the first and second substrates 3', 4' in thismanner, the substrates are connected together by applying pressure topress the substrates together whilst at the same time applying a heatbonding treatment, at a temperature of about 150-180° C. for a period oftime of about 1 hour for example, so as to cause the spacer walls 14 onthe second substrate 4' to bond to the first alignment layer 7' on thefirst substrate 3'. This further heat treatment will result in changingof the surface alignment properties, and particularly the pretilt angleξ, of both alignment layers 7', 8', although the surface alignmentproperties of the first alignment layer 7' will be changed to a greaterextent than the surface alignment properties of the second alignmentlayer 8' due to the fact that the first alignment layer 7' has beencured to a lesser extent.

In such a method in accordance with the invention, this difference inthe extent to which the surface alignment properties of the alignmentlayers 7', 8' are changed by such further heat treatment, and also byany subsequent application of heat during filling of the cell 1 withliquid crystal material 2 or subsequent to such filling, may becompensated for by utilising different materials for the first andsecond alignment layers 7', 8', for example a material initially havinga smaller pretilt angle ξ for the first alignment layer 7' and amaterial initially having a greater pretilt angle ξ for the secondalignment layer 8', or vice versa.

Whilst it is preferred to utilise different materials for the first andsecond alignment layers 7', 8' to enable the surface alignmentproperties of the two layers to be tuned by the subsequent processingsteps applied in order to ensure that the two alignment layers havesubstantially similar surface alignment properties, and particularlysimilar pretilt angles ξ, in the manufactured device, it is alsopossible for such tuning to be effected by utilising alignment layers ofsimilar materials which are subsequently processed in different ways toprovide alignment layers having similar surface alignment properties inthe manufactured device.

What is claimed is:
 1. A method of manufacturing a liquid crystal devicewhich comprises a cell including a layer of liquid crystal materialcontained between two substrates each of which is provided with analignment layer for determining the surface alignment of the moleculesin the liquid crystal material, the method including the steps of:(a)applying a first alignment layer to a first substrate; (b) applying asecond alignment layer to a second substrate; (c) heat treating thefirst and second substrates to cure the first and second alignmentlayers; (d) rubbing the first and second alignment layers to provide therequired surface alignment; (e) connecting the first and secondsubstrates together so that the first and second alignment layers faceone another with a gap therebetween; and (f) filling the cell byintroducing liquid crystal material between the first and secondsubstrates; wherein, in step (c), different heat treatments havingdifferent effects on the surface alignment properties of the first andsecond alignment layers are applied to the first and second substrates,and the first and second alignment layers are made of differentmaterials and/or are differently processed in order to compensate forthe effects of the different heat treatments so that the first andsecond alignment layers have similar surface alignment properties in themanufactured device.
 2. A method according to claim 1, wherein the firstand second alignment layers are made of different materials and/or aredifferently processed so as to ensure that the first and secondalignment layers have pretilt angles ξ within 0.5° of one another in themanufactured device.
 3. A method according to claim 1, wherein the firstand second alignment layers are polymerised by the heat treatments ofstep (c).
 4. A method according to claim 1, for manufacturing a colourliquid crystal device in which the first substrate incorporates a colourfilter layer, wherein, in step (c), a relatively low temperature heattreatment is applied to the first substrate and a relatively hightemperature heat treatment is applied to the second substrate.
 5. Amethod according to claim 4, wherein the relatively low temperature heattreatment is at a temperature less than 180° C., and the relatively hightemperature heat treatment is at a temperature substantially greaterthan 180° C., for example at a temperature of approximately 300° C.
 6. Amethod according to claim 4, wherein a material is selected for thefirst alignment layer which initially has a smaller pretilt angle ξ thanthe material selected for the second alignment layer.
 7. A methodaccording to claim 4, wherein a material is selected for the firstalignment layer which initially has a greater pretilt angle ξ than thematerial selected for the second alignment layer.
 8. A method accordingto claim 1, wherein spacers are applied to the second substrate prior toconnecting the first and second substrates together so that, when thefirst and second substrates are connected together, the spacers serve tospace the first and second substrates apart.
 9. A liquid crystal devicemanufactured by a method according to claim 1.